Showing papers in "Chromatographia in 1994"
TL;DR: In this article, the authors show that the linear flow through a packed capillary in electrochromatography is independent of the particle diameter, and since the driving force (electroosmosis) produces no hydrostatic pressure, it is possible to use very small diameter particles and consequently generate large theoretical plate values often>300,000 plates per metre with reduced plate heights <1.
Abstract: Electrochromatography couples the very high efficiencies associated with capillary zone electrophoresis, with reversed-phase liquid chromatography (RPLC). Therefore, electrochromatography couples the differences in electrophoretic mobilities of related species, with the modification of selectivities that are available to users of RPLC through variation of the stationary and mobile phases. Electrochromatography uses an electric field to drive the mobile phase through a packed capillary, and because the resulting flow profile is plug-like, very high efficiencies are achieved. Also, the linear flow through a packed capillary in electrochromatography is independent of the particle diameter, and since the driving force (electroosmosis) produces no hydrostatic pressure, it is possible to use very small diameter particles and consequently generate large theoretical plate values often>300,000 plates per metre with reduced plate heights<1. The capabilities of electrochromatography are demontrated by means of the analysis of several drug compounds.
258 citations
TL;DR: In this article, a detailed investigation of these effects using capillaries cooled by natural convection has confirmed that self heating of the capillary is indeed largely responsible, and the extent of self heating has been determined by three independent methods and to a first approximation is proportional to the power dissipation in the Capillary.
Abstract: In isothermal CE the migration velocity of analytes and the number of theoretical plates delivered are expected to be proportional to the field strength. In reality ohmic heating of the capillary causes distortions: the migration velocity increases more rapidly while the plate count increases less rapidly, and may even fall at high values of the field. These distortions are worse the larger the bore of the capillary and the higher the concentration of buffer. A detailed investigation of these effects using capillaries cooled by natural convection has confirmed that self heating of the capillary is indeed largely responsible. The extent of self heating has been determined by three independent methods and to a first approximation is proportional to the power dissipation in the capillary. Decreasing viscosity with temperature is responsible for the nonlinearity of the dependence of velocity upon field strength while increase in the diffusion coefficient of analytes is responsible for the poorer than expected performance at high field strengths.
166 citations
TL;DR: In this article, the authors investigated the dependency of the electroosmotic flow on the fractional volume of acetonitrile in the separation buffer and presented a method that differentiates the contributions to band broadening in CEC.
Abstract: Capillary electrochromatography (CEC) is a technique which uses electroosmotic flow for the propulsion of the mobile phase in liquid chromatography. Packed capillary columns with 100 μm I.D. and octadecyl-silica gel as packing material (particle diameter=3 μm and 5 μm) were produced and employed in CEC. The dependency of the electroosmotic flow on the fractional volume of acetonitrile in the separation buffer was studied. In-column detection was performed with an UV-detector. Plate numbers between 20 000 and 44 000 were achieved. A method is presented that differentiates the several contributions to band broadening in CEC. The instrumental band broadening contribution was found to be excessive. This contribution was attributed to inhomogeneities in the electroosmotic flow during injection.
156 citations
TL;DR: Free D-amino acids (D-AA) were detected as native constituents in juices of vegetables (cultivars of cabbage, tomato, carrot, garlic) and fruits (organes, clementine, grapefruit, lemon, apples, pear, grapes) using gas chromatography or high-performance liquid chromatography (LC) as mentioned in this paper.
Abstract: Free D-amino acids (D-AA) were detected as native constituents in juices of vegetables (cultivars of cabbage, tomato, carrot, garlic) and fruits (organes, clementine, grapefruit, lemon, apples, pear, grapes) using gas chromatography (GC) or high-performance liquid chromatography (LC). For investigation by GC, AA enantiomers were converted into theirN(O)-pentafluoropropionyl 2-propyl esters and resolved on a Chirasil-L-Val capillary column. For determination by LC, precolumn derivatization of AA enantiomers usingo-phthaldialdehyde together with the chiral thiolsN-isobutyryl-L-cysteine orN-isobutyryl-D-cysteine and fluorescence detection of the diastereomeric isoindole derivatives, resolvable on an octadecylsilyl stationary phase, were used. D-Ala (0.6–3.8%) was detected in all freshly pressed plant juices usually in the highest relative amounts. Other D-AA detected were D-Asx (0.1–1.9%), D-Glx (0–1.3%), D-Ser (0–1.7%), D-Arg (0.4–1.2%, in grapes, orange, grapefruit, and clementine) and D-Leu and D-Val (1% in cabbage). Absolute amounts of native D-AA were totally 28–57 μmol L−1 in fruit juices, 14.5 μmol L−1 in a tomato juice and 8.5 μmol L−1 in a carrot juice.
73 citations
TL;DR: In this article, the Plackett-Burman experimental design has been studied in the chiral separation of clenbuterol using capillary electrophoresis (CE) using saturated fractional design approach to simultaneously investigate the variables pH, cyclodextrin concentration, electrolyte ionic strength, methanol concentration, and injection time, each at three levels.
Abstract: Plackett-Burman experimental design has been studied in the chiral separation of clenbuterol using capillary electrophoresis (CE). This saturated fractional design approach was used to simultaneously investigate the variables pH, cyclodextrin concentration, electrolyte ionic strength, methanol concentration, and injection time, each at three levels. Percentage main effects of each variable on resolution, retention time and peak efficiency were calculated. Ionic strength, pH and β-cyclodextrin concentration were found to exert the greatest effect on resolution, pH on retention time and pH and ionic strength on peak efficiency. Results are in agreement with those from a previous univariate study. It is concluded that Plackett-Burman experimental design offers of rapid means for testing robustness of chiral CE methods or for the simultaneous screening of experimental variables.
64 citations
TL;DR: The dependence of the temperature excess, θ, in the capillary bore on the applied power, EI, is considered for both natural and forced convective cooling, using classical heat equations as mentioned in this paper.
Abstract: The dependence of the temperature excess, θ, in the capillary bore on the applied power, EI, is considered for both natural and forced convective cooling, using classical heat equations. The dependence of θ on EI is found to be linear for forced convection but not for natural convection. Use of forced convective cooling and capillaries of large outer diameter reduces θ. Direct comparison of the performance of different systems can be achieved by consideration of θ. Column performance is ultimately limited by thermal gradients across the capillary bore.
63 citations
TL;DR: The method was applied to the quantitative determination of paracetamol levels in capsules and the results obtained agreed well between companies and with both HPLC data and the label claim.
Abstract: Successful transfer of an MECC method for the determination of paracetamol between 7 companies is shown. The method was applied to the quantitative determination of paracetamol levels in capsules. The results obtained agreed well between companies and with both HPLC data and the label claim. Acceptable method performance in terms of relative migration time precision and response factor precision was reported by each company.
58 citations
TL;DR: In this article, a method suitable for the determination of 19 biogenic amines in wine has been developed, which involves derivatization of amines by treatment with dansyl chloride and solid-phase extraction of the derivatives.
Abstract: A method suitable for the determination of 19 biogenic amines in wine has been developed. The method involves derivatization of amines by treatment with dansyl chloride and solid-phase extraction of the derivatives. Prior to the derivatization procedure, clean-up of the wine sample with polyvinylpyrrolidone is carried out. Reversed-phase gradient elution HPLC with UV detection at 250 nm was used to determine these compounds. Some consideration was given to the effect of temperature on the separation process. Linearity of derivatization was obtained for amounts of all the biogenic amines ranging from 500 μg·L−1 to 20 mg·L−1. Limits of detection (signal-to-noise ratio=3) of the amines were similar for all the dansylderivatives (between 50 and 150 μg·L−1). Addition of standard amines was used for the determination of amine recoveries. These were better than 85% for ethanolamine, tryptamine, phenetylamine, putrescine, cadaverine and histamine. The overall process was succesfully applied to identify and quantify biogenic amines in white and red wines.
57 citations
TL;DR: In this article, the authors compared the results of CE with those achieved by HPLC to assess CE as an alternative technique for the determination of honey flavonoids, and concluded that CE could be an alternative approach in honey analysis and particularly in the study of its geographical and floral origin.
Abstract: The general objective is to provide an alternative methodology based on capillary electrophoresis (CE) to characterize flavonoids from honey and hence determine its botanical origin. The specific objective is to compare the separation of flavonoids by CE with those achieved by HPLC to assess CE as an alternative technique for the determination of honey flavonoids. Fourteen different flavonoids isolated from honey were analysed by MECC and compared to the HPLC separations. It was difficult to find specific experimental conditions to separate all the flavonoids from honey in a single MEKC run. Three chromatographic conditions are optimized and, depending on the flavonoid markers sought in honey, the appropriate detection method should be chosen. Compared to the HPLC results, it is clear that CE could be an alternative technique in honey flavonoids analysis and particularly in the study of its geographical and floral origin.
57 citations
TL;DR: In this paper, the authors evaluated the planarity and shape selectivity of two liquid-crystal bonded phases using polycyclic aromatic hydrocarbons (PAHs) as the probe samples in reversed-phase HPLC.
Abstract: The chromatographic retention behaviour of two liquidcrystal bonded phases have been evaluated using polycyclic aromatic hydrocarbons (PAHs) as the probe samples in reversed-phase high performance liquid chromatography (RP-HPLC). The results clearly indicate that these phases have better planarity and shape recognition capabilities than commercially-avaialble polymeric octadecylsilica (ODS) phases whose strong planarity and shape selectivities were found earlier. It can also be concluded from the chromatographic observations that the shape recognition capability of these phases is dependent on both mobile phase composition and column temperature, but that the effect of mobile phase and temperature on the shape selectivity work independently. The retention behaviour can be explained by changes in the phase structure with changes of eluent composition and temperature.
55 citations
TL;DR: Improved HPLC and ESMS conditions have been established, allowing the separation and analysis of oligodesoxyribonucleotides by coupled HPLC-ESMS.
Abstract: Improved HPLC and ESMS conditions have been established, allowing the separation and analysis of oligodesoxyribonucleotides by coupled HPLC-ESMS.
TL;DR: In this article, a process for bonding a monolayer of silane to the surface of alumina, zirconia, thoria and titania by reaction with the hydrolysis product of triethoxysilane is described.
Abstract: A process for bonding a monolayer of silane to the surface of alumina, zirconia, thoria and titania by reaction with the hydrolysis product of triethoxysilane is described. The hydride containing material is then further reacted with a terminal olefin in the presence of a platinum catalyst to produce a chemically modified oxide surface which could be used as a chromatographic stationary phase. The success of this reaction is monitored by diffuse reflectance infrared Fourier transform and cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy as well as by differential scanning calorimetry. On all four oxides both the hydride intermediate and the alkyl bonded product are successfully formed via the silanization/hydrosilation reaction sequence.
TL;DR: Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono-and trifunctional methoxyor ethoxysilanes.
Abstract: Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.29Si and13C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.
TL;DR: The enantioseparation of amphetamine, methamphetamine and various ring-substituted amphetamines by use of a chiral stationary phase carrying immobilized native β-cyclodextrin (β-CyD) selectors is reported and a selection of those methods best suited for each single amphetamine is given.
Abstract: The enantioseparation of amphetamine, methamphetamine and various ring-substituted amphetamines by use of a chiral stationary phase carrying immobilized native β-cyclodextrin (β-CyD) selectors is reported. The system is evaluated for resolving the specified compounds directly without any derivatization and after derivatization with phenyl isothiocyanate (PITC), naphthyl isothiocyanate (NITC) and 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC). This direct enantioseparation is compared with the features of indirect separation of diasteromeric derivatives after reaction with the optically pure Marfey's reagent employing a simple non-chiral alkyl-silica (RP-8) column. A selection of those methods best suited for each single amphetamine is given.
TL;DR: In this article, experimental designs were used in the assessment of a capillary electrophoresis method used to determine drug related impurities, and a fractional factorial screening design was used to identify the critical parameters which were found to be pH and voltage.
Abstract: Robustness testing of capillary electrophoresis methods is an important part of method validation. Appropriate use of experimental designs can be employed in this robustness testing. In this study experimental designs were used in the assessment of a capillary electrophoresis method used to determine drug related impurities. Initially a fractional factorial screening design was used to identify the critical parameters which were found to be pH and voltage. A central composite design was then performed to evaluate the response surfaces for pH and voltage which showed operation at the optimum pH and voltage values.
TL;DR: In this article, a GC method is described for the determination of pollutants in aqueous samples by direct enrichment out of large sample volumes in a PTV injector, where vaporized water is eliminated through the split vent and the analytes are concentrated on an adsorbent inside the insert of the PTV.
Abstract: A GC method is described for the determination of pollutants in aqueous samples by direct enrichment out of large sample volumes in a PTV injector The vaporized water is eliminated through the split vent and the analytes are concentrated on an adsorbent inside the insert of the PTV For the selection of suitable adsorbents, breakthrough volumes of selected pesticides and nitroaromatics on Tenax TA, Tenax GR, Chemipack C18 and graphitized carbon blacks have been determined by experiment The determination of the breakthrough volumes was carried out directly in the PTV insert Tenax TA proved to be the best sorbent with regard to the breakthrough volumes and its inertness to the analytes to be determined As the result of the investigations, 500 μl of aqueous standard solutions of pesticides and nitroaromatics (concn=002 2 μg/l) were analyzed with an average relative standard deviation of 10% The procedure was successfully applied for the analysis of real samples
TL;DR: In this paper, a combination of dynamic headspace and simultaneous steam distillation-extraction techniques were used to isolate volatile components of Parmesan cheese, including 3-(methylthio)propanal, δ-tetradecalactone, 9-tetra-decenoic and 9-hexadecenoic acids.
Abstract: Dynamic headspace and simultaneous steam distillation-extraction techniques were used to isolate volatile components of Parmesan cheese. Identification of the substances was carried out by GC and GC-MS; 110 compounds were identified in the samples obtained using the headspace technique, 105 in the extracts; among them, about 50 compounds were isolated with both procedures. Mass spectral data showed evidence for a number of newly reported compounds such as 3-(methylthio)propanal, δ-tetradecalactone, 9-tetra-decenoic and 9-hexadecenoic acids. Mass spectra of some compounds are discussed and a comparison between the results obtained with the two sampling methods is given.
TL;DR: In this article, the behavior of different layers and solvents was studied in an applied electric field of up to 2000 V cm−1, and the selectivity of separation of nonionic and ionogenic compounds was greatly enhanced.
Abstract: Planar electrochromatographic separations of test substances were performed on non pre-wetted, commercially available, thin-layer plates. The behavior of different layers and solvents was studied in an applied electric field of up to 2000 V cm−1. Evident electrokinetic effects, electroosmosis and electrophoresis were observed only on silica gel and polyamide layers developed with polar solvents. The selectivity of separation of nonionic and ionogenic compounds was greatly enhanced. Although experimental conditions were controlled to a certain extent, results obtained with the same solvents were reproducible within 5%.
TL;DR: High resolution was demonstrated by the separation of hemoglobin variants F and A with pI difference of 0.05 pH units, and by the resolution of the isoforms of an anti-carcinoembryonic antigen monoclonal antibody.
Abstract: An isoelectric focusing (IEF) method in the capillary format with wide linear pH range (pH 3–10) and high resolution has been developed for separations of proteins. The methodology involves applying pressure and voltage simultaneously during the mobilization step of the IEF process, to elute the focused protein zones for detection. Capillary isoelectric focusing (CIEF) is performed in neutral, hydrophilic, coated capillaries of 50 μm I.D., using various concentrations of methyl cellulose to reduce the distortion of focused zone by parabolic laminar flow. A linear relationship between the retention time and isoelectric point (pI) of protein standards was observed. The high resolution of this technique was demonstrated by the separation of hemoglobin variants F and A with pI difference of 0.05 pH units, and by the resolution of the isoforms of an anti-carcinoembryonic antigen monoclonal antibody.
TL;DR: In this article, a theoretical study of the use of rectangular columns in capillary electrophoresis (CE) is presented, employing some of the most important parameters that normally define the performance of CE separations, i.e. thermal effect, analysis speed, efficiency and sample capacity.
Abstract: A theoretical study of the use of rectangular columns in capillary electrophoresis (CE) is presented. It was done employing some of the most important parameters that normally define the performance of CE separations, i.e. thermal effect, analysis speed, efficiency and sample capacity. Theoretical results from rectangular and cylindrical capillaries are compared in terms of the aforementionated parameters. Also, an estimate of the additional zone broadening that arises from the noninfinite dimension in the y-direction, in which the channel has its largest dimension, is presented.
TL;DR: In this paper, the authors show that the limiting condition for avoidance of this undesirable effect requires that the ramp-up rate for the applied voltage is below a critical value, which is given to a good approximation by a simple formula.
Abstract: The application of a high voltage, V, in capillary electroseparations following sample injection, can cause loss of analyte if the rate of thermal expansion of the liquid in the capillary (due to ohmic heating) is more rapid than the rate of electro-migration of the slowest moving analyte into the column. We show that the limiting condition for avoidance of this undesirable effect requires that the ramp-up rate for the applied voltage is below a critical value. This critical (maximum) value is given to a good approximation by a simple formula (Eq. (30)). Limiting values of dV/dt are in the region of 1000 V s−1 when the power loss in the capillary is around 3 W m−1 (e.g. with a field of 30,000 V m−1 and a current of 100 μA). A detailed mathematical analysis which takes full account of the thermal dependence of key variables, indicates that thermal explosion will occur at fields above a critical value (Eq. (21)). We recommend that commercial CES instrumentation incorporates manual or software led ramp-up voltage control.
TL;DR: In this paper, a general method for the analysis of lipid classes by liquid chromatography has been developed using a multivariate optimization strategy to target optimal system conditions, the method was validated using a ruggedness test in the form of a Plackett-Burman design thereby exploring the immediate region around the optimum to ensure stable analytical conditions.
Abstract: A general method for the analysis of lipid classes by liquid chromatography has been developed using a multivariate optimization strategy to target optimal system conditions. The method was validated using a ruggedness test in the form of a Plackett-Burman design thereby exploring the immediate region around the optimum to ensure stable analytical conditions. Detection and quantitation were optimized by a factorial design in the light scattering detector parameters thus ensuring maximum detector response. This method was found to be suitable for a broad range of lipid sources from vegetable and animal origin, examples of the separation achieved are given for oat kernel, soybean and bovine milk lipids.
TL;DR: In this article, a comparative study of the solid phase extraction of phenolic compounds in water was carried out and three different sorbents, C18, CH and PLRP-S, were tested for the preconcentration process in order to determine phenolic compound in water at μg l−1 levels by reversed-phase LC separation and diode-array detection.
Abstract: A comparative study of the solid phase extraction of phenolic compounds in water was carried out. Three different sorbents, C18, CH and PLRP-S, were tested for the preconcentration process in order to determine phenolic compounds in water at μg l−1 levels by reversed-phase LC separation and diode-array detection. The addition of an ion-pair reagent in the extraction process was also studied in order to increase the breakthrough volume of some compounds. Using the optimized conditions the limit of detection was less than 0.5 μg l−1, the maximum admissible concentration (MAC) in drinking water. The method was applied to determine phenolic compounds in Ebro river water however no phenols were found within the detection limits of the method.
TL;DR: CE has been shown to be a useful technique for the quantitative determination of common drug counterions and is now in routine use for batch release of selected drug substance material.
Abstract: CE has been shown to be a useful technique for the quantitative determination of common drug counterions. Data generated by CE compares well with theoretical ion contents, microanalysis and ion chromatography data. Acceptable method performance in terms of linearity, accuracy, precision of peak areas and migration times for the CE methods are demonstrated. An internal standard was shown to improve injection precision. Given their speed, simplicity and robustness, the CE methods described are now in routine use for batch release of selected drug substance material.
TL;DR: In this article, supercritical CO2 modified with methanol at 250 bars and 50°C was required to quantitatively extract Atrazine and 2-hydroxyatrazine from spiked C18-silica.
Abstract: Several investigations were performed to optimise the extraction of polar triazine herbicides using supercritical fluid extraction from two different solid matrices: C18-silica and soil samples. Supercritical CO2 modified with methanol [10% (V∶V)] at 250 bars and 50°C was required to quantitatively extract Atrazine and 2-Hydroxyatrazine from spiked C18-silica. Extraction of Desisopropyl-desethyl-2-hydroxyatrazine (MET) attained only 52%, even following addition of water to the polar modifier. Extractions of spiked soil samples (20 ppm of each pesticide) were successful at 300 bars and 65°C. A non polar wash improved the recoveries of the three target analytes [i.e. atrazine: 88%; 2-hydroxyatrazine: 96%; MET: 41%]. The extraction parameters employed are discussed in this paper.
TL;DR: In this article, the enantiomeric ratios of five monoterpene hydrocarbons were estimated by GC-GC and GCGC-MS using a non-chiral pre-column and a chiral main column.
Abstract: Needles of the Macedonian Pine,Pinus peuce Griseb., a conifer growing in the Balkan mountains, were extracted by a Likens-Nickerson simultaneous distillation-solvent extraction device. The components of this essential oil were identified by the retention times obtained on two columns differing in polarity and by GC-MS. Because of the complex nature of such plant extracts, a multidimensional GC method for enantiomer analysis, using a non-chiral pre-column and a chiral main column has been developed. The enantiomeric ratios of five monoterpene hydrocarbons were estimated by GC-GC and GC-GC-MS.
TL;DR: In this article, the baseline separation of 23 dansylated amino acids (including 3 didanylated species) was reported, and the analysis temperature and fine control of the surfactant concentration were essential in obtaining this result.
Abstract: The baseline separation of 23 dansylated amino acids (including 3 didansylated species) is reported. Lowering of the analysis temperature and fine control of the surfactant concentration were essential in obtaining this result. Calibration graphs with good linearity were obtained.
TL;DR: It has been shown that the theoretical limit of resolution and speed in any velocity-based separation of close solute pairs can not be enhanced by distance-based velocity gradients of any kind.
Abstract: Previously [11], the theoretical limit ofresolution in one class of chromatographic conditions was found and its proof was briefly outlined. Here, the theoretical limits ofresolution andspeed of analysis inany velocity-based separation of mixtures in a linear (nonoverloading) medium have been found. Rigorous proof of the limits is given. It has been shown that chromatographic focusing by negative velocity gradientscannot enhance resolution or speed of analysis beyond theoretical limits available without the focusing. On the other hand, the focusingcan improve these characteristics if they were previously deteriorated due to non-ideal conditions of separation. However, the improvements can not go beyond the recovery of the losses of the otherwise deteriorated separation. In a broader sense, it has been shown that the theoretical limit of resolution and speed in any velocity-based separation of close solute pairs can not be enhanced by distance-based velocity gradients of any kind.
TL;DR: Capillary electrophoresis of the quaternary ammonium ion herbicides, paraquat, diquat and difenzoquat was studied in this paper, where electrolyte running solutions with different alkaline salts were used to reduce adsorption of the solutes on the capillary wall.
Abstract: Capillary electrophoresis of the quaternary ammonium ion herbicides, paraquat, diquat and difenzoquat was studied. Electrolyte running solutions with different alkaline salts were used to reduce adsorption of the solutes on the capillary wall. The influence of pH, temperature, applied voltage, buffer cation and injection mode was investigated. The results are discussed in terms of separation parameters such as migration time, apparent mobility, plate height and resolution. In addition, improvement of sensitivity related to the migration velocity and injection mode was observed. Resolution of paraquat and diquat was critical and required well-established conditions.
TL;DR: Fluorinated bonded silica has been evaluated as a packing material in reversed-phase liquid chromatography (RPLC) with polycyclic aromatic hydrocarbons, chlorinated benzenes and substituted benzenses as sample probes as discussed by the authors.
Abstract: Fluorinated bonded silica has been evaluated as a packing material in reversed-phase liquid chromatography (RPLC) with polycyclic aromatic hydrocarbons, chlorinated benzenes and substituted benzenes as sample probes. The bonded material is useful as a reversed-phase stationary phase and for polar compounds it offers a specific selectivity not found with octadecylsilica (ODS) phases. The phase also retains non-planar compounds more than planar molecules. The results can be explained by conformational differences between ODS and the fluorinated bonded phase by MM2 calculation.